1. Field of the Invention
This invention relates generally to electrical plugs and/or connectors and more particularly to indicating if a phase of an AC power plug or connector is missing.
2. Background of the Invention
Extension cords having a plug at one end and a connector at the other end of an insulated cable having conductors sized to carry a maximum current, are typically used both indoors and outdoors. When used as a general purpose conductor, the extension cord may be used to supply power to a power tool, a window mounted air conditioner, a mobile home etc. When used for a specific purpose such as for an industrial application, it may be used to supply power, on a temporary of semi-temporary basis to a motor for driving industrial equipment welding equipment, etc.
Two phases are frequently used for large domestic appliances such as cooking ranges, water heaters, etc. These can be either split phase (from a center tapped transformer or two phases from a three-phase system. Industrial connectors normally use three-phase rather than single phase electrical power. Such multi-phase systems have several advantages in that they provide a better ratio of cable diameter to maximum voltage, allow for greater voltage on trunk lines than is fed to single phase appliances, although split phase can also achieve this, and permit large motors to operate more efficiently. When the current is supplied via an extension cord, the plug and connector of the extension cord can have two or three separate phase terminals plus an earth terminal and, in some cases, a neutral terminal for a total of five terminals. In some instances, plugs and connectors with two live terminals and a neutral may also be used. Almost all three phase power plugs and connectors have an earth or “ground” connection, but may not have a neutral terminal because large appliances such as circular saws and air conditioners are usually connected to a delta source of power. Such delta connected plugs and connectors have only four terminals, an earth terminal, and X, Y and Z phase terminals. An example of a connector having a neutral terminal is a 30 amp and 20 amp plug which has five pins; an earth, a neutral, and X, Y, and Z phase pins.
Prior to connecting an electrical load such as a three phase motor to a source of electricity, it is important that the user be aware if one or more of the phases is absent. For example, a three phase motor will run when one of the three phases is not present. However when this happens, the currents in the two remaining phase conductors will operate the motor, but they will increase to the point where the motor may be damaged or dangerous overheating may occur. What is needed is an AC power phase detector located in a plug and/or connector which can indicate to a user when one or more phases of a three phase Delta or Wye power supply is absent.